Revert "[dart:js_interop] Export some package:js annotations and check them"

This reverts commit ef7d8d3f9ae55d1cb065fdf8f8b3eb0b7235138a.

Reason for revert: Breaks Dart -> Engine roll?

See:
https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20linux_web_engine/1821/overview
https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/666824/overview

Original change's description:
> [dart:js_interop] Export some package:js annotations and check them
>
> To disallow package:js on dart2wasm, we need to export @staticInterop
> and @anonymous through dart:js_interop. However, users should not mix
> the two @JS annotations, so we add some static checks for that. We also
> export @JSExport to support export classes with dart:js_interop.
>
> CoreLibraryReviewExempt: Reexporting annotations through backend-specific library.
> Change-Id: I54610965da332320170d991d785192341049607e
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293500
> Reviewed-by: Joshua Litt <joshualitt@google.com>
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
> Commit-Queue: Srujan Gaddam <srujzs@google.com>

Change-Id: I2af0755e054a1b0452a5fc617a93257209703793
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293961
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Zach Anderson <zra@google.com>
Reviewed-by: Zach Anderson <zra@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>